2.5Unit 2: Cells, Development, Biodiversity and Conservation

SNAB Biodiversity — IAL Edexcel Linear Revision Notes & Topical Questions

10 min read 12 parts Lesson 7 of 26 1 quiz

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Lesson summary

2.5 Biodiversity — IAL EDEXCEL LINEAR notes: Describe levels of biodiversity and how it is measured, the causes of evolution and speciation, classification, and conservation methods.Biodiversity = the variety of genes, species and habitats in a region, which makes ecosystems resi…

Ecosystem (habitat) diversity
Species diversity
Genetic diversity
Cause of pressure
Habitat destruction
Overexploitation and hunting

Part 1 of 10Levels of biodiversity and endemism

Biodiversity

Biodiversity the variety of genes, species and habitats in a region, which makes ecosystems resilient to environmental change.

Ecosystem (habitat) diversity

Ecosystem (habitat) diversity the range of different habitats in an area.
A coral reef with many microhabitats and niches has high diversity, whereas a large sandy desert has low diversity.

Species diversity

Species diversity the number of species and the abundance of each.
- (species richness)
Greater richness and more even abundance give higher diversity, and diverse ecosystems are more stable.
A forest dominated by one or two tree species can collapse if a pathogen attacks a dominant species.

Genetic diversity

Genetic diversity the variety of alleles and genes in a species, which helps populations adapt to biotic and abiotic changes.
Small or isolated populations have low diversity because inbreeding increases homozygosity, so recessive genetic diseases become more common.

Endemism

Endemism endemic species occur naturally in only one area,
- such as pygmy three-toed sloths on one island off Panama,
- and are at high risk of extinction.

Part 2 of 10Human threats to biodiversity

Human threats to biodiversity

Cause of pressure

Cause of pressure exponential human population growth,
from higher birth rate (more food) and lower death rate, increases demand for land, water, wood and fuel.
- (medicine, hygiene)

Habitat destruction

Habitat destruction clearing land for agriculture, industry, housing, transport and waste causes habitat loss and fragmentation.
Fragmented populations suffer inbreeding or local extinction.
Deforestation is highly damaging because forests have the highest biodiversity.
Marine habitats are destroyed by dynamite fishing of coral reefs and by trawling of sea beds.

Overexploitation and hunting

Overexploitation and hunting resources are used faster than they are replaced, for example unsustainable deforestation, overfished stocks and hunting for bush meat.
Overfishing affects marine mammals and seabirds that eat the fish.

Modern agriculture

Modern agriculture monoculture, removal of hedgerows and walls, draining of wetlands and ponds,
- and heavy use of fertilisers and pesticides reduce biodiversity.
Fertilisers leach into waterways and cause eutrophication.
Insecticides kill non-target species such as pollinating bees.

Climate change

Climate change more extreme weather and rapid warming move species towards the poles or higher altitudes,
- where they may compete badly or be out-competed.
Plants may not move fast enough and may go extinct.
Dissolved CO₂ lowers seawater pH (ocean acidification), harming organisms with calcium carbonate shells.
Warm seas cause coral bleaching, killing coral reefs and the biodiversity that depends on them.

Part 3 of 10Measuring species diversity in a habitat

Species richness

Species richness the number of species in a community, the simplest measure of diversity.

It can mislead because it ignores abundance, so two areas with four species each differ if one is dominated by a single species.
Conservationists prefer an index of diversity, which includes species number and evenness.

Sampling

Sampling large ecosystems cannot be fully counted, so random samples estimate species numbers.
Quadrats (for example 0.25 m² or 1 m²) are placed at random points chosen with a numbered grid and random numbers to avoid bias.
Abundance or percentage cover is recorded, and total cover can exceed or fall below 100%.
Sweep nets catch flying insects and insects in long grass.
Pitfall traps buried in the ground catch ground-dwelling invertebrates.
Kick sampling catches freshwater invertebrates: the stream bed upstream of a net is churned for a set time.

Reliability

Reliability take as many samples as possible and use the same method when comparing habitats.

Part 4 of 10Genetic diversity and the index of diversity

Genetic diversity

Genetic diversity measured from the variety of phenotypes, from DNA sequencing of alleles, or from the heterozygosity index.

Heterozygosity index (H)

Heterozygosity index (H) number of heterozygotes ÷ number of individuals in the population.

A higher H means greater genetic diversity.
H can be found for many loci and averaged to estimate diversity of the whole genome.

Index of diversity D

N(N − 1) ÷ Σn(n − 1).

N is the total number of organisms in the community and n is the number of organisms of one species.
Calculate N(N − 1), then n(n − 1) for each species and add them, then divide.
A larger D means higher diversity, so D is used to compare habitats.

Part 5 of 10Niches and adaptations

Niche

Niche the role of a species in its habitat,

including biotic interactions and abiotic interactions (such as gas exchange).
- (what it eats and what eats it)
Each species has its own unique niche.
Two species in the same niche compete, so one is out-competed, moves to a different niche or goes extinct.
Warbler species in the same conifers coexist by feeding at different heights.

Adaptations

Adaptations features that increase survival and reproduction, and a species must be adapted to both biotic and abiotic factors to hold its niche.
Anatomical adaptations are structural, such as horns, claws and thick fur.
Physiological adaptations are internal processes, such as venom or concentrated urine.
Behavioural adaptations include courtship and defence, or being nocturnal.

Part 6 of 10Natural selection

Variation

Variation it exists within a species and arises from random mutations that introduce new alleles.

Selection pressure

Selection pressure predation, disease and competition mean not all individuals survive.

Natural selection

Natural selection individuals with favourable phenotypes are more likely to survive and pass on their alleles,

so advantageous alleles increase in frequency over generations.
Individuals without the advantageous alleles are less likely to reproduce, so those alleles decrease in frequency.
Changes in allele frequency over generations lead to evolution.

Evidence

Darwin proposed the theory and evidence has strengthened it, such as the peppered moth.

Part 7 of 10Hardy-Weinberg equation

Principle

Principle allele frequencies stay the same between generations if mating is random, the population is infinitely large and there is no migration, mutation or selection.
If frequencies change, migration, mutation or natural selection has occurred.

Equations

Equations p + q = 1 and p² + 2pq + q² = 1.
p is the frequency of the dominant allele and q the frequency of the recessive allele.
p² is homozygous dominant, 2pq is heterozygous and q² is homozygous recessive.

Take the proportion showing the recessive phenotype as q².

Find q by taking the square root.

Find p

1 − q.

Calculate p² and 2pq and check that the three genotype frequencies add up to 1.

Part 8 of 10Reproductive isolation and speciation

Reproductive isolation and speciation

Species

Species organisms sharing characteristics that can produce fertile offspring.

Reproductive isolation

Reproductive isolation allele or phenotype changes stop some individuals breeding with the rest of the population.
Seasonal isolationdifferent mating or flowering times.
Mechanical isolationreproductive parts no longer match.
Behavioural isolationcourtship behaviour no longer attracts mates.

Speciation by geographical isolation

Speciation by geographical isolation a natural or man-made barrier such as water, mountains or a motorway separates populations,
- so there is no gene exchange.
Different selection pressures and random mutations change allele frequencies in each population.
Over a long time the populations differ physiologically, behaviourally and anatomically and become separate species.

Part 9 of 10Classification

Classification

Taxonomy

Taxonomy taxonomy groups species by evolutionary relationships in a hierarchy with no overlap between taxa.
The ranks are domain, kingdom, phylum, class, order, family, genus and species.
Higher ranks contain more organisms with less similarity.

Binomial system

Binomial system a species name is the genus (capital letter) then the species (lower case), both in italics, for example Homo sapiens.
It identifies species universally.

Early classification

Early classification it used appearance, which can mislead because unrelated organisms may look similar, such as whales and sharks.
DNA sequencing and molecular phylogeny now compare genotypes, and more similar molecules mean a more recent common ancestor.

Kingdom	Features
Prokaryotae	Prokaryotic cells with no nucleus, typically less than 5 µm
Protoctista	Eukaryotic, mostly single-celled or simple groups in water; protozoa have no walls and algae have cellulose walls and chloroplasts
Fungi	Eukaryotic saprotrophs or parasites with chitin cell walls, spores and hyphae
Plantae	Multicellular autotrophs with cellulose walls, large vacuoles and chloroplasts
Animalia	Multicellular heterotrophs with no cell walls

Three-domain system
- analysis of RNA genes showed prokaryotes split into Archaea and Bacteria, with Eukarya as the third domain.
- Archaea and bacteria are less closely related than first thought.

Part 10 of 10Conservation of biodiversity

In situ conservation

In situ conservation keeping species in their natural habitat with all their support systems, as in national and marine parks.

Ex situ conservation

Ex situ conservation endangered species are kept in captivity in zoos, botanic gardens, frozen zoos and seed banks.

Seed banks

Seed banks seeds are dried and stored in controlled conditions, collected from several sites to keep genetic diversity.
Seeds are regrown periodically to renew the stock, and the Svalbard Global Seed Vault in Norway holds nearly a million samples.
Seeds of coffee and cocoa cannot be frozen, so generations are grown or tissue culture is used.
Advantagesa stock for research and for medicinal and crop plants, less pressure on wild populations, and education.
Disadvantagea small, genetically limited sample may not represent wild populations.

Zoos

Zoos captive breeding releases offspring into the wild and allows research on genetics, behaviour and needs.
The oryx was saved from extinction and reintroduced to Africa, whereas no captive panda has been reintroduced after more than 60 years.
Problemssmall captive populations lose genetic diversity, some species will not breed in captivity, some zoos cannot provide suitable habitats and keeping animals raises ethical questions.
Animals in zoos may behave differently, so some data are less reliable.

Reintroduction

Reintroduction it prevents extinction in the wild and helps restore habitats and the organisms that depend on the species.
Risksreintroduced organisms may carry new diseases or lack skills to find food or communicate.

Key terms

Term	Meaning
Biodiversity	The variety of genes, species and habitats in a region.
Endemic species	A species found naturally in only one area.
Niche	The role of a species within its habitat.
Natural selection	Survival and reproduction of individuals with favourable phenotypes, increasing advantageous alleles.
Heterozygosity index	Number of heterozygotes divided by number of individuals in a population.
Speciation	Formation of new species from isolated populations.
Binomial	Two-part scientific name made of genus and species.
Ex situ conservation	Conserving species outside their natural habitat, as in zoos and seed banks.

Avoid these mistakes

Treating species richness as the full measure of diversity — it ignores how many individuals each species has.

Starting Hardy-Weinberg calculations from the dominant phenotype — start from the recessive phenotype, whose genotype is known.

Saying individuals evolve — populations evolve as allele frequencies change.

Writing a binomial without italics or with a capital species name — the genus is capitalised and the species is not.

Last updated: 18 October 2026